This invention relates to solder bonding, and in particular, to a method and solder compositions for kinetically controlled solder bonding of parts. The invention particularly applies to the bonding of multiple components to a single substrate such as those used in optoelectronic and electronic hybrid integrated circuits.
Optoelectronic and electronic hybrid integrated circuits are key components of modern telecommunication systems. These circuits typically comprise various circuit elements which are precisely aligned and bonded to a single substrate with high strength, electrically and thermally conductive bonds. Some circuit elements such as laser chips must be aligned to submicron tolerances.
Soldering is the method often used for achieving these conductive bonds. Soldering involves placing a metal mixture between the circuit element and the substrate, melting the mixture to allow it to intimately contact the circuit element and the substrate, and resolidifying the mixture to cause it to bond the circuit element to the substrate. Resolidification of the mixture is typically achieved by lowering the temperature of the circuit element, substrate and mixture. This causes the mixture to freeze and adhere to the circuit element and the substrate. In order to maintain the requisite spatial alignments, each circuit element is individually placed, precisely aligned, and heated to solder the element to the substrate.
Unfortunately, the heat applied during soldering of a subsequent circuit element may cause an earlier soldered circuit element to become misaligned or dislodged because the solder remelts. Solder remelting may also occur during subsequent circuit processing which involves heating of the substrate.
Solder remelting may be reduced or substantially eliminated by using a kinetically controlled solder bond. The assignee herein in U.S. patent application Ser. No. 08/955,686 filed on Oct. 22, 1997, by Coult et al. describes a method and compositions for achieving a kinetically controlled solder bond. Coult et al. uses a binary gold-tin (Auxe2x80x94Sn) solder composition for bonding the circuit elements to the substrate, a quenching layer which freezes the solder bonds produced by the binary solder, and a barrier layer that delays the freezing action of the quenching layer for a selected period of time. The quenching layer comprises one of the two metallic elements of the binary solder and the barrier layer comprises a third metallic element, such as platinum (Pt), which remains a minority component of the mixture. Accordingly, an Au5Sn intermetallic solder bond compound is formed between the circuit elements and the substrate. Because the barrier layer forms only a minority component of the of the final solder bond composition, 5 parts of Au must be used for each part of Sn.
The method and solder compositions of Coult et al. are satisfactory for many applications. However, there are applications that require more efficient use of metallic materials and allow debonding of components from the substrate. Accordingly, there is a need for an improved method and solder compositions for kinetically controlled solder bonding parts which utilize materials more efficiently and permit part debonding.
An improved method for kinetically controlled part bonding comprises applying at least a first chemical element layer of an intermetallic compound to a first part and applying at least a second chemical element layer of the intermetallic compound to a second part. The first and second parts are placed together so that the chemical element layers contact each other. The parts are heated from a storage temperature to a bonding temperature which is slightly above a first melting temperature that melts the chemical element layer of one of the first and second parts into a liquid mixture. The composition of liquid mixture varies with time during heating due to the formation of the intermetallic compound therein by progressive incorporation of the other one of the first and second chemical layers into the mixture. The melting temperature of the liquid mixture increases with time as the composition changes until the melting temperature of the liquid mixture is about equal to the bonding temperature thereby soldifying the liquid mixture into a bond. The parts are then held at a holding temperature which is higher than the storage temperature to maintain diffusion of the other one of the first and second chemical layers into the bond. This forms a quantity of the intermetallic compound in the bond which is sufficient to raise the melting temperature of the bond to a desired usage temperature that is substantially above the first melting temperature. The intermetallic compound is typically a ternary intermetallic compound.
The method and solder composition is especially useful for sequentially bonding multiple device elements to a single optoelectronic circuit support substrate.